Whole organ transplantation remains the only clinically effective method of treating both acute liver failure an specific genetic defects of liver function which result in liver failure. However, there are several factors which limit the application of live transplantation in the treatment of liver insufficiency. These include: limited donor availability, inability to obtain an organ at the time when it is urgently needed, need for life- long immunosuppression and high cost. The purpose of our proposed work is to use isolated normal hepatocytes to: A. Develop Novel Methods of Cell Transplantation. B. Develop A Bioartificial Liver (BAL). Cell Transplantation. Isolated hepatocytes will be sued primarily to treat specific genetic liver defects. In these instances, correction of the defect results in normal liver function. Potential advantages of use of isolated hepatocyte transplantation include: a. cell cryopreservation, b. in vitro cell manipulation to either decrease immunogenicity or correct genetic defects using gene transfer technology cells from single donor can be used to treat multiple recipients, and d. simple technique at potentially low cost. In the past five years we have carried out studies which resulted in the development of successful methods of intraperitoneal transplantation and we plan to continue these studies and specifically address two major limitations of this technique: a. lack of long-term function and b. need for immunosuppression. In addition, we will specifically study hepatocyte transfection in vitro, examining the fate and stability of transfected hepatocytes following transplantation and in vivo by developing techniques of in situ hepatocyte transfection. Bioartificial Liver. The BAL will be used primarily to treat acute liver failure. In the past five years we have developed and tested a BAL system which utilizes microcarrier-attached normal hepatocytes on the outer surface of a hollow-fiber bioreactor. We plan to continue working with this system to define its effectiveness and characteristics, to improve its function and, in addition, develop and test other novel BAL systems. The major limitation of these systems remains inadequate tissue availability. We will thus work towards developing a BAL which utilizes xenogeneic hepatocytes.